// Promise.resolve('hello').then(str => str + 'lagou').then(str => (str += 'I love U') && console.log(str));

Promise.resolve().then(function() {
    return new Promise((resolve) => {
        setTimeout(() => {
            var a = 'hello';
            resolve(a)
        }, 10)
    })
}).then((str) => {
    return new Promise((resolve) => {
        setTimeout( () => {
            var b = "lagou";
            resolve(str + b);
        }, 10)
    })
}).then(str => {
    return new Promise((resolve) => {
        setTimeout(() => {
            var c = 'I love U';
            resolve(str += c);
        }, 10)
    })
}).then(str => console.log(str));